1. Field of the Invention
The present invention relates to an optical fiber capable of compensating both the chromatic dispersion and dispersion slope of a positive-dispersion optical fiber, an optical transmission line constructed by splicing the optical fiber and a positive-dispersion optical fiber, and an optical communications system incorporating the optical transmission line.
2. Description of the Background Art
The wavelength division multiplexing (WDM) optical communications system multiplexes a plurality of component signals having mutually different wavelengths with an optical transmitter to transmit them. A signal lightwave composed of the component signals travels over an optical transmission line to arrive at an optical receiver. The WDM communications system can transmit and receive a large volume of information at a high rate. The conventional WDM communications system uses the C-band (1,530 to 1,565 nm) as a wavelength band for signal lightwaves. To increase the transmission capacity further, the use of the L-band (1,565 to 1,625 nm) is also being studied as the wavelength band for signal lightwaves.
It is desirable that the optical transmission line for the WDM communications system have a small absolute value of the chromatic dispersion over the entire wavelength band of the signal lightwave in order to transmit the signal lightwave with high quality. However, it is difficult to reduce the absolute value of the chromatic dispersion over the entire wavelength band of the signal lightwave when the optical transmission line is constructed by using only one type of optical fiber. Consequently, an optical transmission line is usually constructed by using two types of optical fibers to reduce the absolute value of the average chromatic dispersion of the entire optical transmission line over the wavelength band of the signal lightwave.
For example, published Japanese patent application H6-11620 has disclosed an optical transmission line for the application where the C-band is used as the wavelength band of the signal lightwave. The optical transmission line is constructed by splicing a standard single-mode optical fiber and a dispersion-compensating optical fiber. Here, the single-mode optical fiber is a positive-dispersion optical fiber having a zero dispersion wavelength at a wavelength of 1.3 μm or so, and a positive chromatic dispersion of 15 ps/nm/km or so and a positive dispersion slope of 0.06 ps/nm2/km or so at a wavelength of 1.55 μm. The dispersion-compensating optical fiber has a negative chromatic dispersion and a negative dispersion slope at a wavelength of 1.55 μm, and compensates both the chromatic dispersion and dispersion slope of the single-mode optical fiber.
U.S. Pat. No. 5,838,867 has disclosed another optical transmission line. The optical transmission line is constructed by splicing a non-zero dispersion-shifted fiber and a dispersion-compensating optical fiber. Here, the non-zero dispersion-shifted fiber is a positive-dispersion optical fiber having a zero-dispersion wavelength shifted to a wavelength of more than 1.3 μm, and a positive chromatic dispersion between 1 and 10 ps/nm/km and a positive dispersion slope at a wavelength of 1.55 μm. The dispersion-compensating optical fiber has a negative chromatic dispersion and a negative dispersion slope at a wavelength of 1.55 μm, and compensates both the chromatic dispersion and dispersion slope of the dispersion-shifted optical fiber.
The foregoing optical transmission lines constructed by splicing a positive-dispersion optical fiber (a single-mode optical fiber or a non-zero dispersion-shifted fiber) and a dispersion-compensating optical fiber has a small absolute value of the chromatic dispersion over the entire C-band and can perform high-quality WDM optical communication. However, the absolute value of the chromatic dispersion is not sufficiently small in the L-band. Therefore, they are not suitable for performing WDM optical communication in the L-band.
S. Bigo, et al. have reported in ECOC'99, PD 2-9 (p. 40) a non-zero dispersion-shifted fiber having a chromatic dispersion of 8 ps/nm/km and a dispersion slope of 0.057 ps/nm2/km at a wavelength of 1,550 nm (a chromatic dispersion of 10.3 ps/nm/km and a dispersion slope of 0.06 ps/nm2/km at a wavelength of 1,590 nm). D. W. Peckham, et al. have reported in ECOC'98 (p. 139) a non-zero dispersion-shifted fiber having a chromatic dispersion of 3.7 ps/nm/km and a dispersion slope of 0.0457 ps/nm2/km at a wavelength of 1,550 nm (a chromatic dispersion of 5.5 ps/nm/km and a dispersion slope of 0.046 ps/nm2/km at a wavelength of 1,590 nm). Published international application WO 00/65387 has disclosed a non-zero dispersion-shifted fiber having a chromatic dispersion of +7.7 ps/nm/km and a dispersion slope of +0.08 ps/nm2/km at a wavelength of 1,590 nm. However, no report has so far been published on a dispersion-compensating optical fiber capable of compensating in the L-band both the chromatic dispersion and dispersion slope of the above-described non-zero dispersion-shifted fibers.